1. Field of the Invention
This invention relates to a diamond tool, in particular, a diamond cutting tool, a diamond dresser used for forming and dressing a grinding wheel, or a wire drawing die and more particularly, it is concerned with a synthetic diamond single crystal cutting tool used for precision machining or super-precision machining for cutting nonferrous metals such as aluminum and copper and soft materials such as plastics with high precision.
2. Description of the Prior Art
Natural diamond or synthetic diamond has been used as a diamond single crystal for tools, e.g. precision or super-precision cutting tools, and such diamond contains nitrogen as an impurity but no boron.
In the precision machining or super-precision machining for cutting nonferrous metals such as aluminum and copper or soft materials such as plastics with high precision using a diamond cutting tool, there arises a problem that the diamond cutting tool tends to wear and its life is short. During cutting, the edge part of the cutting tool wears so that the surface smoothness of the cut of a workpiece is deteriorated and the cutting resistance is increased, thus shortening the life of the cutting tool.
When a workpiece 1 of a soft material is cut by a diamond cutting tool 2 as shown in FIG. 5, a cut chip 4 slides continuously at a high speed in contact with a rake face 3 of the diamond cutting tool 2 and the rake face 3 is partially oxidized by the friction heat generated during the same time and the heat of the chip 4 itself which is heated at about 300.degree. to 400.degree. C., to produce a crater depth 5 as shown in FIG. 6. If the crater depth 5 is increased, it is hard for the chip 4 to slide on the rake face 3 and this tends to scratch the surface of the workpiece, thus resulting in deterioration of the surface smoothness thereof.
An edge part 7 of the diamond cutting tool 2 is also worn by the heat of friction with the workpiece 1 resulting in mechanical wearing as shown by 8 in FIG. 7, so that the cutting resistance is increased and a desired cutting precision is hardly obtained. In diamond dressers and wire drawing dies, similarly, a desired shape precision or wire drawing precision is hardly obtained by such wearing.
On the other hand, synthesis of a large-sized diamond single crystal used as a diamond cutting tool has generally been carried out by the temperature gradient method wherein a single crystal is grown on seed crystal, as disclosed, for example, in U.S. Pat. Nos. 3,297,407, 4,034,066 and 4,073,380 and Japanese Patent Laid-Open Publication No. 88289/1977. It has been known for a long time that the diamond single crystals synthesized by these methods contain nitrogen as an impurity from raw materials.
A technique of adding boron to synthetic diamond single crystals has hitherto been disclosed in U.S. Pat. Nos. 4,042,673 and 4,082,185. In this case, however, the addition of boron is carried out for the purpose of converting diamond into an n-type semiconductor or yielding blue color diamond for decoration, but no study as to the addition of boron has been made in the relationship with cutting properties in the field of tools.
Furthermore, there has been proposed a diamond single crystal whose resistivity is lowered by adding boron so as to provide a diamond tool in such a manner as to precisely work the diamond single crystal in a desired shape with a high efficiency through discharge working (Japanese Patent Laid-Open Publication No. 126003/1983), but this is not directed to improvement of the mechanical properties.
In the case of cutting a workpiece of aluminum, copper or plastics by a diamond cutting tool, for example, wearing of the edge part of the cutting tool has hitherto occurred during cutting so that the surface smoothness of the workpiece cut is deteriorated and the cutting resistance is increased, thus shortening the life of the cutting tool. Diamond dressers or wire drawing dies have similarly lost their useful life through wearing.